1. Field of the Invention
The present invention relates to a rotary encoder and, more particularly, to a rotary encoder wherein a radial diffraction grating is mounted on a rotary object, a beam from, e.g., a laser is radiated onto the diffraction grating, and a rotational state represented by a rotational speed, a rate of change in rotational speed, and a rotational angle of the diffraction grating or the rotary object is photoelectrically detected utilizing light diffracted by the diffraction grating.
2. Related Background Art
Conventional photoelectric rotary encoders have been used as means for detecting rotational speeds, rotational angles, and rates of changes in rotational speeds of rotary objects or mechanisms such as computer equipment (e.g., floppy disk drives), office equipment (e.g., printers), NC machine tools, and VTR capstan motors and rotary drums.
A typical example of the conventional photoelectric rotary encoder employs a so-called index scale system, as shown in FIG. 1. In this rotary encoder, a so-called main scale 31 and a so-called stationary index scale 32 are interposed between light-emitting means 33 and light-receiving means 34. The main scale 31 has light-transmitting and light-shielding areas formed at the peripheral portion of a disk 35 at equal angular intervals. The disk 35 is connected to a rotating shaft 30. The index scale 32 has light-transmitting and light-receiving areas at the same equal angular intervals as those of the main scale 31. According to this system, upon rotation of the main scale 31, signals synchronized with the intervals between the light-transmitting and light-receiving areas of the scales 31 and 32 are obtained from the light-receiving means 34. The frequency of the resultant signals is analyzed to detect a rate of change in rotational speed of the rotating shaft. For this reason, when the intervals between the light-transmitting and light-receiving areas of the scales 31 and 32 are reduced, detection precision can be improved. When the intervals between the light-transmitting and light-receiving areas of the scales 31 and 32 are reduced, however, diffraction occurs in the scales 31 and 32. An S/N ratio of an output signal from the light-receiving means is degraded by the influence of diffracted light, and detection precision is undesirably degraded. If the numbers of the light-transmitting and light-receiving areas of the main scale 31 are determined and the intervals between the light-transmitting and light-receiving areas are increased so as not to receive the influence of the diffracted light, the diameter of the disk of the main scale 31 must be increased and the encoder itself becomes thick and bulky in order to obtain stability of the scale. As a result, the rotary object is undesirably overloaded.
U.S. Pat. Nos. 3,726,595 and 3,738,758 describe conventional linear encoders. According to these encoders, a coherent beam is radiated onto the diffraction grating mounted on a moving object, beams of predetermined orders output from the diffraction grating are caused to overlap and form interference fringes, the fringes are detected by a light-receiving means, and intensity of the fringes on the light-receiving surface is photoelectrically converted upon movement of the moving object, thereby obtaining an electrical signal (pulses) and hence detecting a displacement of the moving object.
When the interference fringe detection scheme in this linear encoder is applied to a rotary encoder, all the disadvantages of the conventional rotary encoder are assumed to be solved. However, when the above scheme is actually applied to the rotary encoder, a radial grating as a diffraction grating is formed on a rotary object such as a disk to constitute a scale, and a coherent beam is radiated onto the radial grating of the scale. Since the center of the radial grating is not accurately aligned with the center of the rotary object, an eccentricity often causes a measurement error.
Light components diffracted and output from a plurality of positions of the radial grating are allowed to effectively interfere with each other in order to reduce the influence of the eccentricity. However, if the thickness in the direction parallel to an axis of rotation (parallelism) of the scale comprising a rotary object such as a disk varies, a difference between the optical paths of the diffracted components to be interfered occurs. As a result, a measurement error may occur again.